Large volumes of natural gas (i.e., primarily methane) are located in remote areas of the world. This gas has significant value if it can be economically transported to market. Natural gas (“NG”) is routinely transported from an onshore LNG production plant to another location in its liquid state as liquefied natural gas (“LNG”) by way of loading the LNG in the cryogenic storage tanks of purpose built large ocean going vessels known as “LNG Carriers”. Liquefaction of the natural gas makes it more economical to transport as LNG occupies only about 1/600th of the volume than the same amount of natural gas does in its gaseous state. Prior to liquefaction, raw natural gas that has been sourced from a wellhead is subjected to a series of gas pre-treatment processes including acid gas removal and dehydration to remove contaminants. After liquefaction, LNG is typically stored in cryogenic storage tanks at the LNG production plant either at or slightly above atmospheric pressure at a temperature of around −160 degrees Celsius.
Gas pre-treatment, liquefaction and storage are typically undertaken at a fixed onshore LNG production plant associated with a jetty that is built in sufficiently deepwater to allow berthing of the LNG Carriers. To ship liquefied natural gas (LNG) by sea, a way to transfer LNG between the cryogenic storage tanks of the onshore LNG production plant and the cryogenic storage tanks of the LNG Carrier is required. Traditionally, the transfer means has taken the form of an insulated pipe that is laid on an elevated supporting trestle structure between the onshore LNG production plant and the jetty so that the insulated pipe remains at all times above the water line. These prior art transfer facilities include a vapour return line to return boil-off gas to the onshore LNG production plant. After LNG have been loaded into the cryogenic storage tanks of the LNG Carrier vessel for marine transport LNG is regasified before distribution to end users through a pipeline or other distribution network at a temperature and pressure that meets the delivery requirements of the end users.
The cost of LNG storage and offloading facilities has continued to increase through the years and is now a very significant component of the total installed cost for an LNG project. Efforts to reduce this cost have largely been focused on storage tank size optimization and seeking to leverage the economics of scale via increased LNG train capacity size and improvement in LNG berth utilization.
There remains a need to explore alternative designs for LNG storage and offloading facilities.